Invention relates generally to optical pellicles, and more particularly to a protective mask for such a pellicle.
Pellicles play an important role in the fabrication of semiconductor wafers used in integrated semiconductor circuits. As is perhaps best described in U.S. Pat. No. 4,131,363 to Shea, et al., an optical pellicle is comprised of a thin transparent optical membrane which is stretched over a rigid pellicle frame.
In use, the pellicle frame and membrane are placed over a photomask such that the membrane is positioned at a distance spaced from the photomask. In this arrangement, a light source, typically a mercury lamp, shines light through the pellicle membrane, through the photomask below and then onto the wafer positioned below the photomask. These lamps normally exhibit maximum amounts of light at low wavelengths such as 436 and 365 nanometers (nm). Recently, because of a need to etch more IC components into a given wafer area, the use of so-called excimer lasers has become more widespread. Excimer lasers typically emit light in the 248 and 193 nm ranges, often referred to as deep UV (or deep ultraviolet) light. Regardless of the form of light utilized, the wafer is exposed to a light pattern identical to the circuit pattern in the photomask. Small particulate matter and dust which falls onto the pellicle membrane is out of focus from the photomask pattern during this photolithography process, and therefore will not be reproduced in the light pattern on the silicon wafer.
To fabricate a pellicle, a very thin membrane, normally in the range of 0.50 to 3.0 microns, is first formed on a substrate. The membrane is then removed from the substrate and mounted to a frame. Pellicle frames take many shapes, depending upon the photolithography equipment with which the pellicle is ultimately to be used. By way of example, the frames may be circular, rectangular, or square.
The pellicle membrane is typically adhered to the pellicle frame by an adhesive such as double stick tape or glue. In use, the short wavelengths produced by excimer lasers mentioned above, such as 248 and 193 nm, degrade the adhesive. Degradation of this light-sensitive adhesive can lead to adhesive particles falling directly on the photomask or loosening of the membrane from the frame, destroying the evenly tensed surface of the membrane, and destroying the effectiveness of the pellicle system.
It is therefore a general objective of the present invention to provide a pellicle mask which overcomes the drawbacks and limitations of prior art proposals. More specifically, the invention has as its objects the following: (1) to provide a pellicle having a protective mask which shields the light-sensitive adhesive holding the pellicle membrane to the frame, from low wavelength light such as 248 and 193 nm light, thereby preventing light degradation of the adhesive; and (2) to provide a protective mask for a pellicle which leaves a central portion of the pellicle exposed to light so that light can pass through a central portion of the pellicle and a photomask, and thereafter contact a wafer.